The normal perception of sound occurs when sound waves strike the tympanic membrane and cause it to vibrate. These vibrations are transmitted through the three tiny bones in the middle ear (ossicular chain) to the cochlea in the inner ear, which results in electrical impulses being transmitted through the auditory nerve to the brain. Even if the sound conducting mechanisms of the middle ear are functioning perfectly, a hearing loss can be experienced if the inner ear is damaged.
A conventional, "air conduction" hearing aid can sometimes be used to overcome a hearing loss due to inner ear damage (sensorineural loss) and/or hearing loss due to a mild impediment of the sound conducting mechanism of the middle ear. A conventional air conduction hearing aid works by simply amplifying the incoming sound and delivering the amplified sound signal by way of a speaker positioned in the ear canal. This amplified sound simply "overdrives" the ear's sound conducting mechanism.
Since an air conduction hearing aid must have some of its componetry in the ear canal, and since it also requires a fairly normal tympanic membrane and middle ear space, some hearing impaired persons are unable to derive any benefit from a device.
Persons who cannot benefit from an air conduction hearing aid can sometimes benefit from a "bone conduction" hearing aid. A bone conduction hearing aid works by converting the sound signal into a mechanical vibratory stimulus. Heretofore, the vibrating portion of the aid has been placed against the skin, usually behind the ear, under some pressure. The vibrator transmits its vibrations through the skin and soft tissue into the bone structure of the skull. The vibration of the skull stimulates the cochlea and a sound is perceived. Such bone conduction devices are not very popular due to several limitations. First, the devices are bulky and must be worn on a head band or a special eyeglass frame in order to keep the vibrator pressed tightly against the skull. In addition, because the vibration must be transmitted through the soft tissue overlying the skull, the fidelity of sound and the efficiency of the device are poor.
Proposals have been made for improving bone conduction devices for stimulating the inner ear. One such proposal is disclosed in U.S. Pat. No. 3,209,081 in which a radio receiver is implanted underneath the skin and includes a vibration generating means which is connected to the temporal bone subcutaneously. A transmitter may be located at any remote place on the body of the user within the range of the implanted radio receiver for generating a modulated signal in response to sound received by a microphone. This modulated signal is received by the radio receiver and the vibrator is caused to vibrate in response to the modulated signal and set up vibrations within the temporal bone which in turn stimulates the inner ear to create a perception of sound. This implanted radio receiver is quite complex and includes numerous implanted electronic components including a power supply, which are susceptible to malfunction and other potential problems which could cause extreme difficulty due to the implanted nature thereof.
A second proposal relates to some experimental work conducted in Europe and described in a recent published paper wherein a direct bone conduction device was implanted which included a bone screw implanted directly in the temporal bone subcutaneously and a post connected directly thereto. This post extends percutaneously (through the skin) to a location externally of the skin. A vibrator which creates vibrations in response to a modulated signal is connected to this post and vibrations are transmitted by the post to the bone screw and thence to the temporal bone of the skull to stimulate the inner ear and create the perception of sound. This device has distinct disadvantages, not the least of which are the likelihood of infection and the undesirability of a ceramic element extending permanently through the skin from aesthetic, psychological and comfort standpoints.